1. Field of the Invention
The present invention relates generally to orthogonal frequency division multiplexing access (OFDMA), and more particularly, to a method and apparatus for generating an edge sidelobe canceling signal for alleviating interference between users without causing additional bandwidth loss in an uplink, and an uplink communication method and apparatus using the same.
2. Description of the Related Art
Orthogonal frequency division multiplexing (OFDM) is a frequency-efficient modulation method that is robust to a frequency selective fading channel. Recently, OFDM has been positively considered and examined as a candidate technique for the next generation mobile communication. OFDM is a process of dividing a high-speed serial signal into a plurality of low-speed parallel signals and modulating them onto orthogonal subcarriers, respectively, for transmission or reception. Accordingly, the orthogonal subcarriers having a narrow bandwidth undergo flat fading so as to have generally good characteristics for the frequency selective fading channel. In addition, orthogonality between the subcarriers can be kept using a simple method such as insertion of a guard interval by a transmitting terminal, and therefore, a receiving terminal does not require a complex equalizer or a rake receiver in a direct sequence-code division multiplexing access (DS-CDMA) method. Due to these good characteristics, OFDM is employed as a standard modulation method for digital broadcasting, wireless local area network (LAN) such as IEEE 802.11a or HIPERLAN, fixed broadband wireless access, etc. In addition, OFDM has been considered as a candidate technique for modulation/demodulation and multiple access in a universal mobile telecommunications system (UMTS).
To accomplish next generation mobile communication by meeting the rapidly increasing users' demands for, for example, ultrahigh multimedia services, diverse multiple access methods based on OFDM have been researched and developed. Of those diverse multiple access methods, in an OFDMA method, data of each user comprises a subset of orthogonal subcarriers based on OFDM. In other words, among all of the orthogonal subcarriers, M adjacent subcarriers are grouped and defined as a single subband, and a plurality of subbands are allocated to different users. In a communication system employing such an OFDMA method, signals transmitted from user terminals, i.e., mobile stations, to a base station individually undergo an independent frequency offset. Accordingly, even though a receiving terminal, i.e., the base station, accurately estimates and compensates for each user's frequency offset, interference between users occurs.
During an uplink, a guard interval is used to prevent each user frequency offset from interfering with an adjacent user subband. The guard interval, in which MG subcarriers between adjacent subbands are not modulated, reduces interference by an adjacent subcarrier. When many guard intervals are allocated, however, a bandwidth loss increases although a signal-to-interference ratio (SIR) regarding each user frequency offset improves. In addition, when a modulation method of high bandwidth efficiency, such as an m-bit quadrature amplitude modulation (QAM), is employed to make the best use of advantages of an orthogonal multiple subcarrier system, a high SIR is required. Consequently, methods of alleviating interference using a guard interval have a limitation.
More specifically, in conventional OFDMA methods, for example, a method introduced by Concept group Beta [“OFDMA Evaluation Report—The Multiple Access Proposal for the UMTS Terrestrial Radio Air Interface (UTRA),” Tdoc/SMG 896/97, ETSI SMG Meeting No. 24, Madrid, Spain, December 1997], a method introduced by J. van de Beek and P. O. Borjesson et al. [“A Time and Frequency Synchronization Scheme for Multiuser OFDM”, and a method introduced by H. Alikhani, R. Bohnke, and M. Suzuki [BDMA (Band Division Multiple Access—A New Air-Interface for 3rd Generation Mobile System in Europe”, Proc. ACTS Summit, Aalborg, Denmark, October 1997, pp. 482-488), a differential quadrature phase shift keying (DQPSK) modulation method and an 8-DPSK modulation method are used, a subband is composed of 22 through 25 subcarriers, and a guard interval is comprises 2 through 3 subcarriers. Therefore, a bandwidth loss of minimum 7.4% through maximum 12% is incurred. When a modulation method of higher bandwidth efficiency is used, an additional bandwidth loss may cause a decrease in transmission capacity.